The Prior Art
Silver and gold-free palladium-based dental alloys have previously been disclosed in the literature. However, these prior disclosures suffer from various deficiencies. U.S. Pat. No. 4,261,744, issued Apr. 14, 1981, covers a dental alloy composition of 5-10 weight % indium, 5-10.5 weight % tin, up to 7.5 weight % cobalt, chromium or nickel, up to 0.25 weight % silicon and a balance of palladium. The incorporation of silicon in this composition in the given quantity will produce a low melting eutectic phase of silicon and palladium. This phase melts at 800.degree. C. which is lower than the porcelain firing temperature range of 960.degree.-1010.degree. C. Distortion of the restoration may occur during normal porcelain firing due to the lower melting temperature of the eutectic phase. In addition, the low melting temperature eutectic phase will hamper efforts to join restoration sections together by soldering prior to porcelain firing. This "pre-soldering" operation takes place at 1050.degree.-1120.degree. C. and the low melting temperature eutectic phase causes a portion of the alloy to be molten which may easily result in distortion of the prosthesis.
The composition of the above mentioned patent and similar compositions on the market do not contain a grain refiner e.g., ruthenium, iridium or rhenium to assure a single phase homogeneous material devoid of large grains. The presences of such large grains (about 400-600 microns) fosters the phenomena known as "hot tearing" and "marginal flaking". "Hot tearing" means the fracturing of the alloy casting during solidification in the investment. "Marginal flaking" occurs when stress is applied to the edges of the alloy casting and small pieces break off as a result.
U.S. Pat. No. 4,123,262 discloses a non-staining alloy intended for use in the porcelain fused-to-metal technique. However, the alloy composition of the patent requires the expensive gold component for successful results.
U.S. Pat. No. 4,387,072 issued June 7, 1983, discloses a dental alloy comprising by weight 50-85% of palladium, 5-40% copper and/or cobalt, 1-15% gallium, up to 5% of nickel, gold, indium, ruthenium or tin, up to 0.5% rhenium and/or iridium and up to 1% boron. The reference does not teach or suggest the combination and appropriate concentrations of metals to provide an alloy having all of the beneficial properties of the alloy of this invention and it does not disclose the necessity of the formation of the proper oxide for satisfactory adherence of the alloy to porcelain when preparing dental restorations. The alloy product sold under this patent, which contains by weight 79% palladium, 2% gold, 10% copper and 9% gallium, exhibits a eutectic phase and requires the user to heat the alloy in air at 1010.degree. C. for five minutes or until a brown oxide is formed before applying the porcelain. In contrast, the alloys of the present invention are immediately ready for application of porcelain when reaching the porcelain firing temperature range. This results in a substantial savings of time in the preparation of dental restorations.